The important roles of voiding function are urine storage and voiding, which are regulated by a coordinated action of the bladder and the urethra. That is, during the urine storage, the bladder smooth muscle is relaxed and the urethra sphincter is contracted, whereby a state in which urethral resistance is high is maintained, and urinary continence is also maintained. On the other hand, during the voiding, the bladder smooth muscle is contracted while the urethra smooth muscle is relaxed, and the contraction of the external urethral sphincter is also inhibited. Examples of voiding dysfunction include a storage dysfunction such as overactive bladder and the like in which urine cannot be retained during urine storage and a voiding dysfunction in which urine cannot be drained sufficiently due to increase in the urethral resistance and decrease in the bladder contractile force. These two dysfunctions may be expressed simultaneously.
In treatment of a storage dysfunction such as overactive bladder and the like, anticholinergic agents have been used frequently. However, these agents cannot provide a sufficient therapeutic effect, and further, side effects based on the anticholinergic action (dry mouth, gastrointestinal symptoms, eye symptoms, arrhythmias, or the like) appear, and accordingly, administration of the agents may be often interrupted. Further, the anticholinergic agents reduce the bladder contractile force, and are therefore contraindicated for urinary frequency/incontinence accompanying urethral obstruction such as benign prostatic hyperplasia and the like.
Voiding dysfunction is caused by an increase in urethral resistance during the voiding or a decrease in the bladder contractile force. As a disease causing an increase in urethral resistance, voiding dysfunction accompanying benign prostatic hyperplasia is well known, which is characterized by urethral obstruction due to nodular hypertrophy of the prostate tissues. An α1 receptor antagonist has now been used for the purpose of treating voiding dysfunction accompanying benign prostatic hyperplasia (see, for example, Non-Patent Document 1). Other causes of the increase in urethral resistance include functional obstructions such as urethra relaxation failure during voiding or detrusor-external urethral sphincter dyssynergia and the like due to neurological disorders such as diabetes, aging, bone marrow damage, pelvic surgery, and the like,. With patients with these diseases, there exists many cases in which the α1 receptor antagonist is ineffective. On the other hand, a decrease in the bladder contractile force during the voiding, referred to as underactive bladder, acontractile bladder, neurogenic bladder, or the like, also causes voiding dysfunction. Known factors for decreasing the bladder contractile force include aging, neurological diseases such as diabetes, Parkinson's disease, multiple sclerosis and the like, bone marrow damage, and neurological disorders due to pelvic surgery. Examples of an agent for treating a decrease in the bladder contractile force during voiding include bethanechol chloride which is a muscarinic receptor agonist and distigmine bromide which is a cholinesterase inhibitor. Both of these drugs have side effects, and thus, their satisfactoriness is low (see, for example, Non-Patent Documents 2 and 3). In voiding dysfunction caused by an increase in the urethral resistance or a decrease in the bladder contractile force as described above, residual urine after voiding is observed. Increased residual urine may cause a decrease in effective bladder capacity, and thus, cause overactive bladder symptoms such as urinary frequency and the like, or severe symptoms, such as hydronephrosis in some cases, and in this regard, there is a demand for a therapeutic agent which is more effective than a current therapeutic agent.
It is known that a relaxation system due to nitric oxide (NO) is present in the smooth muscle, and NO produced in the nerve terminals or locally activates soluble guanylate cyclase present in the smooth muscle cells. The activated guanylate cyclase increases cyclic guanosine monophosphate (cGMP) in the cells. On the other hand, the cGMP is degraded into 5′-GMP by phosphodiesterase (PDE) which is an enzyme degrading the cGMP. An increase in the intracellular cGMP concentration is considered to contribute significantly to the smooth muscle relaxation. Therefore, the decrease of the NO-cGMP system causes relaxation failure of the smooth muscle. For example, in patients showing urethral obstruction in benign prostatic hyperplasia or in the elderly as described above, it is reported that NO production is significantly decreased (Non-Patent Documents 4 and 5).
As a subtype of PDE which specifically degrades cGMP, PDE5, PDE6 and PDE9 are known, and among these, PDE9 has a higher substrate affinity than PDE5 and PDE6 (Non-Patent Document 6). Further, from the viewpoint that in the distribution of expression in various tissues, PDE9 is observed at its highest expression in the human prostate (Non-Patent Document 7), it plays an important role in smooth muscle relaxation in lower urethra smooth muscle and a PDE9 inhibitor enhances the relaxation of the urethra via the elevation of cGMP in the tissue. Therefore, it is considered that the PDE9 inhibitor exhibits an effect against voiding dysfunction due to an increase in the urethral resistance. Since the PDE9 inhibitor decreases the urethral resistance, an effect against voiding dysfunction in which the bladder contractile forces are decreased can be expected. In addition, the decrease in residual urine due to an improvement of the voiding dysfunction will lead to the improvement of overactive bladder symptoms such as urinary frequency and the like or avoidance of renal disorders. Therefore, it is considered that the PDE9 inhibitor is useful as an agent for preventing and/or treating storage dysfunction, voiding dysfunction, and bladder/urethral diseases.
For example, as a compound having a PDE5- and/or PDE9-inhibitory action(s), in Patent Documents 1 and 2, there are disclosed quinoxaline derivatives represented by the following formulae (A) and (B), respectively. Further, in Patent Documents 3 and 4, there are disclosed a thienopyrimidine derivative and a quinazoline derivative as compounds having a PDE5- and/or PDE9-inhibitory action(s), respectively. In addition, in Patent Documents 5 to 12, there is disclosed a pyrazolopyridine derivative which has a PDE9-inhibitory action.
Furthermore, in Patent Documents 13 to 17, there are disclosed compounds represented by the following formulae (C) to (G), but there is no specific disclosure of the compounds of the present invention. In addition, there is no description that the compound has a PDE9-inhibitory action and can be used for treating disorders in voiding function.

(For the symbols in the formulae, refer to each of the corresponding patent publications.)